In atmospheric CO2 enrichment experiments, nearly all plants almost always exhibit increases in photosynthetic rates and biomass production when environmental conditions are optimal for growth. Even when conditions are less than favorable (low soil moisture, poor soil fertility, high soil salinity, high air temperature), many plants still exhibit a CO2-induced growth enhancement; and that relative or percentage enhancement is sometimes (more often than not, in fact) greater than what it is under ideal growing conditions. It is sometimes suggested,however, that results obtained from CO2-enrichment experiments conducted in growth cabinets, greenhouses and other enclosures may not reflect real-world plant responses to atmospheric CO2 enrichment due to perturbations in microclimate caused by the enclosures. Thus, Free-Air CO2 Enrichment or FACE technology was developed as a means to enrich the air with CO2 around vegetation while having minimal effects on the surrounding microclimate; and the following paragraphs of this summary document describe the results of some of those experiments that were conducted on various grassland species, many of which were growing naturally in pastures.
In the study of Nitschelm et al. (1997)1 , 18-m-diameter circular plots of white clover were established at a field station of the Swiss Federal Institute of Technology near Zurich and exposed to atmospheric CO2 concentrations of 350 and 600 ppm; and after one season of growth, the four researchers were able to report that elevated CO2 increased aboveground biomass production by a whopping 146%. In addition, the extra 250 ppm of CO2 increased carbon inputs to the soil by 50% while decreasing root decomposition by 24%, thereby enhancing the carbon sequestration capacity of the soils in the CO2-enriched plots.
In another Swiss experiment, Luscher et al. (1998)2 studied 9 to 14 genotypes of each of 12 native grassland species collected near Zurich that were transplanted into FACE arrays maintained at atmospheric CO2 concentrations of 350 and 700 ppm. And in doing so, they learned that twice-ambient concentrations of CO2 generally increased aboveground biomass in all twelve species included in the experiment, while showing no preferential effects on any specific genotype of any of the studied species.